Conventionally, there has been a known measurement method for sound field measurement in which an impulse response or transfer function of a sound field space which is a linear time-invariant system to be measured is measured using a measurement signal having a flat frequency spectrum over the entire frequency bands (e.g., patent document 1).
Here, in order to obtain high S/N ratio when performing the sound field measurement, a noise component in the sound field space, i.e., background noise, is required to be taken into account. Typically, the background noise has a frequency spectrum showing that the lower the frequency band, the higher is the energy level of the background noise (see FIG. 10). Here, FIG. 10 shows the frequency spectrum of the background noise measured in a listening room. The frequency spectrum of the background noise shown in FIG. 10 is such that the lower the frequency in a frequency band A, the higher is the energy level, and in a frequency band B, the energy level is substantially fixed. Accordingly, the above-described conventional sound field measuring method has a problem in that an S/N ratio deteriorates in the frequency band A, because the lower the frequency in the frequency band A, the higher is the energy level of the background noise therein.
In order to solve the above problem, a measurement method has been proposed in which such a signal whose frequency spectrum increases by 3 db/Oct from a high frequency band toward a low frequency band as shown in FIG. 11 is used as the measurement signal (refer to, e.g., patent document 2). FIG. 11 shows a frequency spectrum of a conventional measurement signal for which the deterioration of S/N ratio in a low-frequency band has been improved. This measurement signal is generated by sweeping a frequency of a sinusoidal signal in a non-linear (logarithmically proportional) manner such that the frequency changes gradually at a low frequency side and steeply at a high frequency side. By using this measurement signal, a measurement in which an influence from the background noise is reduced particularly in the low frequency band A can be performed.
[Japanese Patent No. 2725838]
[Japanese Patent No. 2867769]